As a method of converting an alternating-current voltage into a direct-current voltage, two methods explained below are generally known.
In a first method, a diode bridge circuit and a smoothing capacitor are used. The diode bridge circuit full-wave rectifies an alternating current supplied from an alternating-current power supply. The smoothing capacitor smoothes a full-wave rectified direct current.
In the first method, an electric current always flows through a series circuit of two diodes irrespective of whether the alternating-current voltage is positive or negative. When the electric current flows, in each of the two diodes, a power loss equivalent to a product of the electric current flowing through the diode and a forward voltage of the diode occurs.
In a second method, a power factor improving converter (PFC) is interposed between the diode bridge circuit and the smoothing capacitor in the first method. The power factor improving converter steps up a direct-current voltage full-wave rectified by the diode bridge circuit.
In the second method, as in the first method, since an electric current flows through the series circuit of the two diodes when the full-wave rectification is performed, a power loss occurs. In addition, since the electric current alternately flows to a field effect transistor (FET) and a diode included in the power factor improving converter, a further power loss occurs.
Since the power factor improving converters needs to convert a waveform of an input current into a sine wave, an output voltage has to be set higher than an input voltage. However, a voltage necessary in a load is not always a voltage higher than the input voltage. In that case, a step-down converter is connected in a post stage of the power factor improving converter. The step-down converter steps down the voltage stepped up by the power factor improving converter to a desired voltage. A power loss also occurs in the step-down. An entire power converting apparatus has a three-stage configuration of AC-DC conversion, DC-DC. (step-up) conversion, and DC-DC (step-down) conversion. The power loss appears as a product of power losses in the three stages. For example, if it is assumed that efficiency per one stage is 0.95, efficiency in the three stages is 0.95×0.95×0.95=0.86. That is, excellent conversion at the efficiency of 95% falls to 86% if the three stages are connected. In this way, even if conversion efficiency of each stage is high, the conversion efficiency is markedly deteriorated in multiple stages.
Recently, in the public, there is an increasing power saving request for electronic apparatuses. At the same time, it is also an essential condition that current harmonic noise is not emitted to prevent an external environment from being adversely affected. Therefore, it is demanded to attain both of conversion efficiency improvement and a current harmonic suppression effect of a power converting apparatus that supplies electric power to a load.
The related art is described in JP-A-2007-110869 and JP-A-2008-295248.